The 28-day limit on holding terror suspects without charge is likely to be doubled by the government.
The police say they need more flexibility over the 28-day limit.
Home Office Minister Tony McNulty said the government wanted to extend the limit, "probably" to 56 days.
Security Minister Lord West said "about 50" was the figure being talked about - but said safeguards would have to be in place to win over critics.
But veteran Labour MP David Winnick accused the government of "trying to find a number out of a hat".
It is thought the government will make an attempt to extend the current 28-day limit in the Counter Terrorism Bill to be announced in the Queen's Speech on Tuesday.
Attempts in 2005 to extend pre-charge detention to 90 days ended in Tony Blair's first Commons defeat as PM.
Instead, MPs voted to extend the period from the then limit of 14 days to 28 days.
Mr Blair warned them he hoped they would not "rue the day" and argued the police case for 90 days had been "compelling".
Lord Carlile, the government's terror legislation watchdog, said that it was likely that in a small number of cases more than 28 days would be needed in the future.
"There is a real risk that an extremely serious terrorism event could take place involving a large number of terrorists in which it would be difficult, if not beyond possible, to carry out all the necessary inquiries within a period of 28 days," he told BBC Radio 4's Today programme.
"If that eventuality is reasonably foreseen, and I foresee it, then in my view we should introduce a very highly protected system that would at least enable those most serious cases to be deal with properly.
"I think it's vital that nobody should be detained for an hour longer than is necessary. But also, nobody should be detained for an hour shorter than is necessary in the over all public interest, subject to the right legal protections."
But Lord West told the BBC: "The 90 days I have no doubt whatsoever, was far too long.
"I think when it was tried to be done it was done in the most appalling way and we need to make sure we don't make that sort of mistake again."
Prime Minister Gordon Brown has said he wants political consensus on the issue and has suggested one option would be to double it to 56 days.
Lord West told the BBC the complexity of some cases meant that "we really do need more time to look into them".
But he admitted he was "not sure" how the government would be able to persuade critics to accept an extension to "around 50 days".
"We have to show absolutely that we really do need this and we have to show absolutely that we have real safeguards in place, certainly judicial oversight, Parliament would have to be told... and there may be other mechanisms we can do to look after people."
Earlier Home Office minister Tony McNulty was also asked about extending the limit.
He told Sky News: "I would think up to 56 days probably, but people need to understand what we are asking for, my view is 14 days plus is extraordinary, 28 days plus is even more extraordinary, so there needs to be real substantial danger and circumstances."
Both the Liberal Democrats and the Conservatives say they will not back any plans to extend the limit unless new evidence comes forward that it is needed.
Questioned recently, shadow home secretary David Davis said there remained "no shred of evidence" to support an extension, which he said could end up curbing civil liberties that "thousands, if not millions, of British citizens have died to defend".